The invention relates to digital controllers that are used to control industrial machines and processes.
A programmable controller typically has a main processor that is connected through one or more I/O interface modules to I/O devices that control a machine or process. In early controllers the main processors and the I/O modules were housed separately, the I/O modules being placed in an I/O rack that was located near the machine or process being controlled. Several of these I/O racks could be placed at different locations and connected through cables to the main processor, however, the length of the cables between the I/O modules and the main processor was limited, because such cables were used for parallel data transmission.
Another type of programmable controller has provided a processor and I/O racks that could communicate over serial transmission lines, to permit the I/O racks to be located more remotely, at destinations up to 10,000 feet away from the main processor. Such a controller is described in Galdun et al, U.S. Pat. No. 4,254,473, issued Mar. 3, 1981, and entitled "Rack Adapter for Serially Connected I/O Interface Racks." This system employs a scanner circuit of an earlier period with a processor-based adapter circuit that allows serial connection of I/O racks along serial data channels. The scanner and adapter transmit words of conventional I/O status data in a synchronized, bit-oriented fashion.
The ability to interface I/O devices with a programmable controller has been recently enhanced with the invention of a programmable controller having remote I/O interface racks that can accommodate intelligent high-density I/O modules. Such a system is disclosed in a copending patent application of Schultz et al, Ser. No. 242,132, entitled "Programmable Controller for Executing Block Transfer with Remote I/O Interface Racks," filed Mar. 9, 1981, and assigned to the assignee of the present invention. Although larger quantities of I/O status data can be transferred in such a system, there is a need to reduce the time of individual transmissions of conventional I/O status data, so that a number of I/O interface racks can be scanned quickly enough to provide the responsive control that is needed in the industrial environment.